Classes use the same pass-by-value semantics as structures. If you pass a class instance to an operation, the class and all its members are passed. The usual type compatibility rules apply: you can pass a derived instance where a base instance is expected. If the receiver has static type knowledge of the actual derived run-time type, it receives the derived instance; otherwise, if the receiver does not have static type knowledge of the derived type, the instance is sliced to the base type. For an example, suppose we have the following definitions:
// In file Clock.ice:
class TimeOfDay {
short hour; // 0 - 23
short minute; // 0 - 59
short second; // 0 - 59
};
interface Clock {
TimeOfDay getTime();
void setTime(TimeOfDay time);
};
// In file DateTime.ice:
#include <Clock.ice>
class DateTime extends TimeOfDay {
short day; // 1 - 31
short month; // 1 - 12
short year; // 1753 onwards
};
Because DateTime is a sub-class of TimeOfDay, the server can return a DateTime instance from getTime, and the client can pass a DateTime instance to setTime. In this case, if both client and server are linked to include the code generated for both Clock.ice and DateTime.ice, they each receive the actual derived DateTime instance, that is, the actual run-time type of the instance is preserved.
Contrast this with the case where the server is linked to include the code generated for both Clock.ice and DateTime.ice, but the client is linked only with the code generated for Clock.ice. In other words, the server understands the type DateTime and can return a DateTime instance from getTime, but the client only understands TimeOfDay. In this case, the derived DateTime instance returned by the server is sliced to its TimeOfDay base type in the client. (The information in the derived part of the instance is simply lost to the client.)
Class hierarchies are useful if you need polymorphic values (instead of polymorphic interfaces). For example:
class Shape {
// Definitions for shapes, such as size, center, etc.
};
class Circle extends Shape {
// Definitions for circles, such as radius...
};
class Rectangle extends Shape {
// Definitions for rectangles, such as width and length...
};
sequence<Shape> ShapeSeq;
interface ShapeProcessor {
void processShapes(ShapeSeq ss);
};
Note the definition of ShapeSeq and its use as a parameter to the processShapes operation: the class hierarchy allows us to pass a polymorphic sequence of shapes (instead of having to define a separate operation for each type of shape).
The receiver of a ShapeSeq can iterate over the elements of the sequence and down-cast each element to its actual run-time type. (The receiver can also ask each element for its type ID to determine its type.)